Radio frequency (RF antennas are used to receive and/or radiate RF signals. An effective antenna for use in transmission will exhibit an acceptably low standing wave ratio (SWR) at the frequencies of interest, and will present a reasonably good impedance match to the output of the transmitter, typically 50Ω to 75Ω. While some antenna designs such as beams exhibit directionality, i.e., more antenna gain in one direction compared to another, in many applications it is desired that the pattern of radiation from the antenna be omni-directional. Further it is often desired that the antenna not require ground radials, as radials undesirably increase antenna wind load, as well as the manufacturing cost. Further, radials diminish robustness of the antenna design, especially in inclement weather.
Many innovations in antenna design have come from the amateur radio community. Pioneer work in the area of so-called fractal antenna has been accomplished by Nathan Cohen (W1IR, W1YW) of Belmont, Mass., e.g., U.S. Pat. Nos. 6,104,349, 6,127,977, 6,140,975, 6,445,352, 7,019,695, and 7,701,396, among others.
Another innovation in antenna design is depicted in FIGS. 1A and 1B, namely the so-called Don Johnson screwdriver antenna, named after its late inventor Don Johnson (W6AAQ) of Esparta, Calif. Overall antenna 10 includes a whip portion 20, typically 3′ to perhaps 8′ in length, mounted to make electrical connection with the upper end of an inductor 30. Inductor 30 typically is formed about a non-conductive cylinder of perhaps 2″ diameter and perhaps 12″ length. The upper portion of housing 40 includes conductive finger stock that presses against inductor 30, effectively grounding to housing 40 all portions of inductor 40 that are within the housing. Inductor 30 and the cylinder it is formed upon can be urged vertically upward and downward within a metal cylinder housing 40 to alter magnitude of the effective inductance protruding from housing 40. A threaded rotatable shaft 80 is connected between the lower end of the inductor 30 cylinder and the rotatable shaft of a small DC motor 70. Motor 70 is typically a motor from an electric screwdriver, hence the “screwdriver” name for the antenna. Two wires from motor 70 can be applied to a plus or a minus polarity DC voltage, to cause motor 70 to rotate clockwise or counterclockwise, causing more or less of inductor 30 to lie within housing 40, which is to say to decrease or increase magnitude of the effective inductance protruding from the housing. A matching inductor 50 is formed at the base of the antenna and a length of typically 50Ω coaxial cable 60 is connected as shown. The other end of coaxial cable 60 will typically go to a transceiver, or transmitter, or receiver. Tuning antenna 10 simply involves applying plus or minus DC voltage to motor 70 to mechanically resonate the antenna to a desired frequency range. Antenna 10 can be tuned by first setting the transceiver (or receiver) to a desired frequency and then adjusting the effective length of inductor 30 by rotating motor 70 in the proper direction (by applying plus or minus voltage to motor 70) to resonate the antenna, as evidenced by a peak in amplitude of received signals.
In FIG. 1A, DC voltage has been applied to motor 70 to rotate nearly all of inductor 30 into housing 40. The effect at RF frequencies is that only the portion of inductor 30 protruding from housing 40 functions as an inductor. The antenna operates as a center loaded device whose resonance is determined primarily by the effective inductance 30 and the whip 20. In FIG. 1B, DC voltage was applied to motor 70 to rotate threaded shaft 80 such that more of inductor 30 can now resonant with whip 20. Clearly the additional effective inductance used in FIG. 1B will lower the resonant frequency of the overall antenna. Advantageously the antenna can operate continuously within a very wide range of frequencies, merely by applying DC voltage to motor 70 to cause more or less inductance to be used. In practice, many thousands of Don Johnson screwdriver antennas have been used worldwide with great success over frequencies ranging from as low as about 3.5 MHz to as high as perhaps 144 MHz.
In other applications, especially higher frequency applications, a less mechanical antenna may be desired, especially for considerations of cost and ease of construction. One common type of antenna, especially for VHF (2 m range wavelengths) and/or UHF (70 cm range wavelengths), is the so-called collinear antenna. A collinear antenna is an array of at least two dipole antennas, configured such that every element of each dipole is an extension, relative to a longitudinal antenna axis, of the other dipoles in the array. Collinear antennas can exhibit gain over an isotropic radiator.
FIG. 2A depicts a collinear antenna 90 comprising collinear elements 100, 110, 120, used with preferably at least four quarter-wavelength radials 130 mounted at the antenna base. Radials 130 function as a ground plane. Preferably coaxial cable 60 is coupled to antenna 90, with the other end of coaxial cable 60 coupled to a transceiver, a transmitter, or a receiver (not shown). Lowermost element 100 in FIG. 2A is a quarter-wavelength at the nominal frequency of interest. Intermediate element 110 is coupled to act as a half-wave delay element, and uppermost radiating element 120 preferably has a length equal to a half-wave. The various elements 100, 110, 120 can be fabricated from lengths of coaxial cable, whose center conductor is indicated by phantom lines, and whose outer shield conductor is indicated by solid lines on either side of the center conductor. Note that the collinear arrangement alternates electrical connection between the center conductor of an element and the outer conductor.
In FIG. 2A, if one tried to use quarter-wavelength element 100 with an extension half-wavelength (i.e., center-conductor to center-conductor, shield-to-shield), no additional gain would result due to phase cancellation of radiation in the quarter-wave and half-wave elements. FIG. 2B depicts voltage amplitude versus phase for the various elements of antenna 90. As confirmed by FIG. 2B, non-radiating half-wave delay element 110 provides the desired ground reference function. This results from coupling the shielded outer conductor of element 100 to the inner conductor of element 110, which inner conductor acts as a ground reference. Note at the base of antenna 90 that radials 130 are also coupled to this ground reference via the center conductor of element 100. As shown in FIG. 2A, after a quarter-wavelength at the junction of elements 100 and 110, the shield and inner conductor are swapped. At its upper end, element 110 is coupled to the lower end of half-wave coaxial element 120, again by swapping of center conductor and shield outer conductor. As the radiated radio frequency energy exits the upper end of element 120 it is back in phase with quarter-wavelength radiating element 100. If desired additional elements, i.e., another triplet of elements 100, 120, 120 could be added atop present uppermost element 120 in collinear fashion. However a point of diminishing returns effectively occurs at about four elements in that marginal further increase in gain does not warrant the cost of the additional elements.
Disadvantageously, antenna 90 requires several, typically at least four, quarter-wavelength radials 130, preferably bent downward at an angle of perhaps 45° to establish an RF ground. As noted, an RF ground reference node exists at the junction of radials 130 and the outer shield of coaxial cable 60. Radials often require machining to properly make good electrical connection at the base of antenna 90. In practice stainless steel radials are preferred for reasons of strength and electrical contact over less expensive aluminum radials. The presence of radials impacts the robustness of the antenna design. Radials can easily break off in the presence of strong winds, or by birds perching on the radials. If the radials are on the ground, they may be damaged from being walked upon. Further, the electrical conductivity between the radials and the shield of coaxial cable 60 will inevitably deteriorate over time.
FIG. 3 depicts an attempt in the prior art to eliminate radials by using a quarter-wave sleeve. Referring to FIG. 3, antenna 140 has at its base a quarter-wave element 100, then a half-wave delay element 110, above which is disposed an upper half-wave radiating element 120. These collinear elements 100, 110, 120 in antenna 140 are configured similarly to the same elements in antenna 90 in FIG. 2A, and are made from segments of coaxial cable. However rather than employ radials (as in FIG. 2A), antenna 90 employs a conductive quarter-wavelength sleeve 150 to implement an effective quarter-wavelength foldback and RF ground reference. The term “foldback” is used in that sleeve 150 covers a portion of the connecting coaxial cable 60. Sleeve 150 is commonly made of conductive brass or copper pipe, and the connection to coaxial cable 60 is typically made within the sleeve. This configuration advantageously gains robustness by eliminating radials, and exhibits a slightly lower angle of radiation that can add to transmitting range of the antenna. However the sleeve configuration can make it difficult to achieve desired low SWR due to inherent coupling between the outer shield conductor of coaxial cable 60 and the wall of sleeve 150.
FIG. 4 depicts yet another attempt in the prior art to implement an omni-directional antenna without radials. As shown in FIG. 4, a portion of this antenna looks like the letter “J”, and this configuration is sometimes referred to as a “Super-J” antenna. A full description of antenna 160 may be found in QST magazine, September 1994, p. 61-62, J. Reynante (W6JRR) “An Easy Dual Band VHF/UHF Antenna.” Referring to FIG. 4, the lower end of antenna 160 is a quarter wavelength matching element, typically 300Ω twinlead 170, whose two leads or wires are shorted together at the bottom 180. The RF impedance at bottom 180 is of course 0Ω, but at a distance Δ above bottom 180, the RF impedance will be close to the impedance of coaxial cable 60 to be a good match, e.g., 50Ω or so. The upper end of quarter-wavelength matching element 170 is coupled to a half-wave radiating element 190, as the upper end of quarter-wavelength matching element 170, and either end of half-wave radiating element 190 are both RF high impedances.
Note that elements 170 and 190 are disposed vertically and will exhibit vertical radiation of RF energy. By contrast, the upper end of half-wave radiating element 190 is coupled to a horizontally disposed delay element 200. Delay element 200 comprises two parallel quarter-wavelength element coupled in a horizontally-disposed “U”-shaped configuration. The horizontally polarized RF energy associated with the lower and with the upper elements of delay element 200 are 90° out-of-phase with respect to each other and thus cancel one another. Ideally the phase delay and radiation patterns associated with element 200 are perfectly out-of-phase, but in practice some phase error and associated antenna inefficiency will exist. “U”-shaped delay element 200 may be thought of as contributing an outgoing lower quarter-wavelength delay and an incoming quarter-wavelength delay. The net result is that these horizontally disposed elements represent an effective half-wave delay element 200. The upper portion of “U”-shaped delay element 200 is connected to the lower end of a vertically disposed (and vertically radiating) half-wave radiating element 210. In this fashion the antenna of FIG. 4 implements the functional equivalent of the ready access to ground that was present in the antenna of FIG. 2A. The desired overall half-wavelength delay with desired non-radiating characteristics for 180° of the phase waveform is achieved by “U”-shaped element 200.
Regrettably, antenna 160 is not robust in that delay element 200 projects out horizontally from the vertical antenna into the environment, and is difficult to reliably fasten between radiating elements 190 and 210. Alternatively some designs also seek to achieve phase delay with inductor-capacitor (LC) components rather than with an element 200. However such solutions are not optimum because losses and tolerance changes in the L and C components vary over time, which can reduce effectiveness of the desired delay function.
FIG. 5 depicts a so-called conventional “J-pole” antenna that can be fabricated from a single length of 300Ω twin lead cable, comprising lead 1 and lead 2, that has a gap or notch cut in one lead (lead 1), and whose bottom region has a short between the two leads. The lower portion of antenna 220 comprises a quarter-wavelength matching element 230, sometimes referred to as a shorting stub element, whose lower end 180 has both leads shorted together to form a low impedance end of the J-pole antenna. A perhaps 0.25″ gap or notch 240 is formed in one side of the matching element (the left side in FIG. 5), cutting through one of the two wires or leads. A half-wave radiating element 250 is connected to the upper portion of quarter-wavelength matching element 230. As shown the upper end of J-pole antenna 220 is open, and is high impedance.
Thus, as used herein, the term “J-pole” antenna is understood to refer to an antenna comprising spaced-apart first and second conductive wires, e.g., twinlead, shorted together at one end to form a zero impedance first end, and a quarter-wavelength away from this zero impedance first end, an open high impedance quarter-wavelength second end. The first conductive wire (lead 1) has a notch or gap cut through the wire approximately a quarter wavelength above the zero impedance first end, and the second conductive wire (lead 2) is approximately three-quarter wavelength at resonant frequency. An RF low impedance feedpoint exists a distance Δ in each lead above the zero impedance first end. The quarter wavelength section adjacent the zero impedance first end functions as a quarter-wavelength impedance matching element, and the half-wavelength of each lead measured from the high impedance second end forms a half-wavelength radiating element.
In a conventional half-wave antenna, the antenna ends are high impedance and the antenna center is low impedance. But in a half-wave vertical dipole antenna such as antenna 220, end matching must be done at a high impedance point. As noted above, this condition is satisfied using quarter-wave shorted stub matching element 230 at the lower end of antenna 220, by having the upper end of half-wave radiating element 250 open, e.g., not shorted or connected to anything. Thus, J-pole antenna 220 exhibits high impedance at the upper end of half-wave radiator element 250, and exhibits 0Ω impedance at shorted region 180. However at a distance Δ above short 180 a good match to typically 50Ω coax feedline 60 may be found. As such, quarter-wavelength matching element 230 acts like an impedance matching transformer. Matching distance Δ is commonly on the order of perhaps 0.5″ or so for an antenna resonant in the 70 cm band. In practice, distance Δ may be determined experimentally using an antenna analyzer. Further details regarding the design of antenna 220 may be found at QST magazine, February 2003, pp 38-401, E. Fong (WB6IQN), “The DBJ-1: A VHF-UHF Dual-Band J-Pole”, and QST magazine, March 2007, E. Fong (WB6IQN), “The DBJ-2: A Portable VHF-UHF Roll-up J-pole Antenna for ARES”.
J-pole antenna 220 is similar to a vertically oriented end-fed half-wave dipole, but with a radiation pattern closer to an ideal vertical dipole. The enhanced radiation pattern results because feedline 60 is in-line rather than perpendicular to the antenna. A well-designed J-pole antenna is a good half-wave radiator that provides about 2.1 dB gain over an isotropic radiator, but no gain relative to a half-wave antenna.
It will be appreciated that J-pole antenna 220 is omni-directional, inexpensive to fabricate, and requires no radials. In practice the antenna can be inserted within a length of UV-resistant PCV pipe that is sealed at the top and bottom, to provide a robust configuration with relatively low wind resistance. In practice J-pole antenna 220 can achieve about a 1.5 dB gain improvement over a quarter-wave ground plane antenna because it is a true half-wave antenna. In a conventional ground plane antenna such as was described in FIG. 2A, ground radials 130 were necessary to act a counter element (e.g., ground or earth). With radials bent downward from say 0° (i.e., horizontal) to about 45°, and disadvantageously a relatively high angle of radiation will result. By contrast, a J-pole such as antenna 220 has no radials and advantageously exhibits a lower angle of radiation, which results in a gain of about 1.5 dB in the horizontal plane relative to a conventional ground plane antenna.
While the J-pole provides an interesting starting point for many antenna designs, further gain may be demanded of many applications. Thus there is a need for an inexpensive, robust antenna with low wind resistance and high reliability. Preferably such antenna should provide gain beyond what a conventional J-pole antenna can provide and indeed should provide gain over a half-wave antenna. Such antenna should be omni-directional, should not require radials or special collars, and should not require an absolute ground connection.
The present invention provides such an antenna.